This invention relates to a method and apparatus for removing contaminants from pellet or granular materials.
In the manufacture of fibers and other products from polymeric materials, these polymeric materials frequently exist at some point in the manufacturing process in the form of beads or pellets.
A pneumatic conveying system is generally used to transport the pellets from one station to another station in the manufacturing process. When pneumatically conveying pellets it is usually necessary to use a high transport velocity to prevent settling and to maintain movement of the pellets. Unfortunately, such a high transport velocity causes frictional heat to build up as the pellets travel and contact a tube or conduit wall, and can cause a melting of some of the pellets. The melted portions of the pellets can adhere to the tube or conduit wall in a thin polymeric layer, which will eventually break loose from the wall, travel with the pellets, and contaminate the process downstream. These thin strips of polymeric material which are deposited on the walls of the tube or conduit by the moving pellets are considered contaminants and are referred to as "snakeskins" if film-like, or "angel hair" if hair-like, or as "bird nests" if hair-like and tangled together.
In a typical commercial fiber plant arrangement, polymer pellets are received in a vacuum receiver, dumped into an extruder feed tank, gravity fed through a line to a proportioner that feeds into an extruder. The contaminants can accumulate at the exit of the extruder feed tank, the entrance and exit of the proportioner, and at any point in any line at which there are valves.
Simple devices such as a screen located at the vacuum receiver exit have been utilized to remove the snake skins or angel hair from the feed. These devices however tend to plug as the polymer pellets tend to bridge and plug the flow. Usually a screen can be used but must be vibrated or pulsed with pneumatic air to prevent the polymer bridging.
It is desirable to provide a simple apparatus that will remove the contaminants from the feed.
It is also desirable to remove these snakeskins, angel hair and bird nests from the polymer pellets in order to obtain a uniform final product.